TM  1-1520-240-10 4-1-2            NOTE Threat symbols (except for U) shown on indi- cator illustrations are for illustration purposes only. Actual threat symbols are classified. f. The indicator displays threat symbols corresponding to  threat  signals  detected  and  identified  by  the  system. Threat relative position from the helicopter is shown on the indicator. Symbol position relative to the center of the indica- tor shows the threats lethality. The highest priority threats (most lethal) are shown nearest the center. The 12 outer edge  markings  on  the  indicator  graticule  represent  clock positions relative to helicopter heading. The system displays a  maximum  of  seven  threat  symbols.  If  the  number  of threats in the environment exceeds the number the system can display, only the seven highest priority threats will be displayed.  If  a  detected  threat  cannot  be  identified  as  a specific threat type, it is displayed as an unknown (symbol U). Search radars and fire control radars operating in search mode are displayed as strobes at the edge of the indicator. The  position  of  the  strobe  on  the  display  represents  the relative bearing of the search radar from the helicopter. New threats appear in boldface on the display. Threats that drop- out of the environment are ghosted on the display for 10 seconds before being dropped. A ghosted symbol appears as though drawn with a dotted line. g. Threats are announced by voice messages over the helicopter ICS. Either of the two voice message for- mats  can  be  selected  using  page  2  of  the  CDU  ASE control  layer.  RWR  AUDIO  1  or  normal  (full message format) selects full audio. RWR AUDIO 2 or terse (short- ened message format) provides shorter audio messages and reduces the audio clutter in dense signal environ- ments. Both modes provide specific threat type or threat position voice messages. h. When dense signal environments cause the sys- tem to operate in a degraded (less sensitive) mode, the system informs the operator by flashing the plus (+) sym- bol   on   the   RSDS   indicator   and   the   voice   message “Threat Detection Degraded” will be heard over the ICS. When the system sensitivity returns to normal, the plus (+)  symbol  in  the  center  of  the  RSDS  indicator  stops flashing and the voice message “Threat Detection Re- stored” will be heard over the ICS. i. The RSDS, LDS, and MWS all execute periodic built-in-test (PBIT) routines during normal operation to verify operational status with the results of these tests being reported to the digital processor. Failures of PBIT for these three systems are indicated by the presence of an “F” replacing  the  plus  (+)  symbol  in  the  center  of  the  RSDS indicator. The “F” informs the operator that one or more of the systems has failed at least some portion of their respec- tive  PBIT  routines.  This  prompts  the  operator  to  perform initiated built-in-test (IBIT) to determine what system(s) has failed and the extent of the failure(s).. j. IBIT may be selected by the operator. If the RSDS interface circuitry within either RTU fails the self-test, the RWR  TEST  FAIL  advisory  is  displayed  on  the  MFD. Faulty receivers are shown on the indicator as blinking symbols and the voice message “APR-39 Failure” will be heard over the ICS. If the test is good, the voice message “APR-39 Operational” will be heard over the ICS. The RWR  FAIL  caution  is  displayed  if  the  RSDS  interface circuitry within both RTU’s fails while in its normal operat- ing state. Self-test of the laser detecting set (LDS) and the missile warning system (MWS) are incorporated in the RSDS self-test. k. The   system   interfaces   with   the   Interference Blanker Unit (IBU) in an attempt to prevent other aircraft systems,  which  operate  in  the  same  approximate  fre- quency range, from interfering with it. The lo-band portion of  the  RSDS  is  blanked  by  the  IBU  when either  the IFF transponder system or the TACAN system is transmitting. The hi-band portion of the system is blanked by the IBU when the radar altimeter system, radar beacon transponder system, multimode radar system, pulse radar jammer sys- tem, or the CW radar jammer system is transmitting. This blanking is accomplished to prevent the possible detection of false threats as a result of interference induced by these systems. Because the RSDS is a passive system, it does not provide any input to the IBU for the potential blanking of other systems.